Optical film with variable angle prisms

ABSTRACT

An optical turning film has a first surface including an array of prisms. The array has a plurality of first prisms, with each of the first prisms having a first prism configuration, and a plurality of second prisms, with each of the second prisms having a second prism configuration different from the first prism configuration. The optical film also has a second surface opposing the first surface, and light rays incident to the first surface are directed by the plurality of first prism and the plurality of second prisms along a direction substantially parallel to a viewing axis.

This application is a division of Ser. No. 09/415,873 filed Oct. 8, 1999which now is U.S. Pat. No. 6,356,391.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to light transmissive optical films, andmore particularly, the invention relates to an optical film with anarray of variable angle prisms.

2. Description of the Related Technology

Backlit display devices, such as liquid crystal display (LCD) devices,commonly use a wedge-shaped lightguide. The wedge-shaped lightguidecouples light from a substantially linear source, such as a cold cathodefluorescent lamp (CCFL), to a substantially planar output. The planaroutput is then coupled to the LCD.

The performance of a display device is often judged by its brightness.From a subjective standpoint relatively small increases in overallbrightness are not easily perceived by the end user of the displaydevice, but it is possible to measure relatively small increases inbrightness objectively. While not directly appreciated by the end user,a display with an objectively measured increase in overall brightness ofonly a small percentage, for example, perhaps as little as 1 percent, isperceived as being significantly better by the designer of the productusing the display. This is because the designer can allocate less powerto the display device, yet still achieve an acceptable level ofbrightness. For battery powered, portable devices, this translates tolonger running times.

The alternatives for increasing display brightness include using more orbrighter light sources. Counter to the ability to decrease the powerallocation to the display device, additional light sources and/orbrighter light sources consume more energy, which for portable devicesthis correlates to decreased battery life. Also, adding light sources tothe device may increase the product cost and can lead to reducedreliability of the device.

Brightness is also enhanced by more efficiently using the light that isavailable within the display device, i.e., to direct more of theavailable light within the display along a preferred viewing axis. Anumber of mechanisms have been employed within display devices toimprove display device efficiency. For example, brightness enhancingfilms having prismatic structures are frequently used to direct lightthat would otherwise not be viewed along the viewing axis. A typicalflat panel display device may use several different films to provide anoverall bright, high contrast display with substantially uniform outputalong the preferred viewing directions. Surface diffusers or bulkdiffusers are sometimes used to mask defects in the output of thelightguide, but most diffusers scatter light from the viewing axis andtherefore reduce on-axis brightness.

Lightguide improvements have also contributed to improved brightness indisplay devices. Typical lightguides extract light by diffusion and maybe enhanced by geometric recycling. Light rays entering the lightguideencounter diffusing elements, typically a pattern of white dots appliedto a surface of the lightguide, and are diffusively extracted byscattering from the lightguide. Other light rays are totally internallyreflected within the lightguide until encountering a diffusing element.Losses are encountered in these processes, and because the light isdiffusely extracted, without any collimation, on-axis brightness islower. With enhancement, the diffuse light rays may be directed more onaxis, in a quasi-collimation process, which results in enhanced on-axisbrightness.

Another method of extracting light from a lightguide is by use offrustrated total internal reflection (TIR). In one type of frustratedTIR the lightguide has a wedge shape, and light rays incident on a thickedge of the lightguide are totally internally reflected until achievingcritical angle relative to the top and bottom surfaces of thelightguide. These sub-critical angle light rays are then extracted, ormore succinctly refract from the lightguide, at a glancing angle to theoutput surface. To be useful for illuminating a display device, theselight rays must then be turned substantially parallel to a viewing, oroutput, axis of the display device. This turning is usually accomplishedusing a turning lens or turning film.

A turning lens or turning film typically includes prism structuresformed on an input surface, and the input surface is disposed adjacentthe lightguide. The light rays exiting the lightguide at the glancingangle, usually less than 30° to the output surface, encounter the prismstructures. The light rays are refracted by a first surface of the prismstructures and are reflected by a second surface of the prism structuressuch that they are directed by the turning lens or film in the desireddirection, e.g., substantially parallel to a viewing axis of thedisplay.

An optical effect resulting from frustrated TIR lightguides notadequately compensated for in presently known display devices isreferred to as ripple. Ripple is a periodic fluctuation in the luminanceof the wedge light output. The fluctuation amplitude, spatial frequencyand phase are largely determined by the wedge angle, input aperture andlamp coupling to the input aperture. The emergence angle and thelocation of emergence from the wedge maps directly onto a position onthe input aperture and an emission angle from the input aperture. Thus,nonuniformities in the spatial and angular components of emission fromthe input aperture map into corresponding changes in the wedge outputbrightness. The result is an effect wherein the display has a bright anddark banded appearance parallel to the light source. The effect is mostperceptible nearest to the lightguide entrance, but may be observed overthe entire output surface.

Attempts to correct ripple, or essentially mask the appearance ofripple, include adding some form of optical power or scattering to thewedge structure to try and provide more uniform light extraction.However, these attempts have not proven entirely satisfactory.

SUMMARY OF THE INVENTION

In accordance with the invention, an optical film useful for reducing oreliminating ripple (luminance non-uniformity) is formed with an array ofturning prisms formed on an input surface. The prisms are containedwithin the array in clusters of prisms that include prisms of two ormore prism configurations.

In a first aspect of the invention, an optical turning film has a firstsurface including an array of prisms. The array has a plurality of firstprisms, with each of the first prisms having a first prismconfiguration, and a plurality of second prisms, with each of the secondprisms having a second prism configuration different from the firstprism configuration. The optical film also has a second surface opposingthe first surface, and light rays directed at a glancing angle to thefirst surface are directed by the plurality of first prisms and theplurality of second prisms through the optical turning film and areemitted from the second surface at an angle substantially parallel to aviewing direction of a display.

In another aspect of the invention, an optical turning film is a sheetof light transmissive optical film having a first surface and a secondsurface. A plurality of first light redirecting prisms and a pluralityof second light redirecting prisms are formed in the first surface. Theplurality of first light redirecting prisms and the plurality of secondlight redirecting prisms are arranged in an array of prisms on the firstsurface. The first light redirecting prisms have a first prismconfiguration, and the second light redirecting prisms have a secondprism configuration, different from the first prism configuration. Theplurality of first light redirecting prisms and the second plurality oflight redirecting prisms are organized into a plurality of first prismgroups and a plurality of second prism groups. The first prism groupsand the second prism groups are arranged in the array in a pattern offirst prisms and second prisms.

In still another aspect of the invention, an illumination device has alightguide with an input surface and an output surface. Light raysincident on the input surface are refracted into the lightguide andpropagate within the lightguide by TIR. These light rays then exit theoutput surface via frustrated TIR at a glancing angle to the outputsurface. A light source is coupled to the input surface to project thelight rays on the input surface, and a turning film is disposed adjacentthe output surface. The turning film has a first surface and a secondsurface opposing the first surface. An array of prisms is formed in thefirst surface of the turning film, and the array of prisms has aplurality of first prisms having a first prism configuration and aplurality of second prisms having a second prism configuration differentfrom the first prism configuration. The array of prisms are arranged toredirect the light rays from the glancing angle to an anglesubstantially aligned with a viewing axis of the illumination device.

BRIEF DESCRIPTION OF THE DRAWINGS

The many advantages and features of the present invention will becomeapparent to one of ordinary skill in the art from the following detaileddescription of several preferred embodiments of the invention withreference to the attached drawings wherein like reference numerals referto like elements throughout and in which:

FIG. 1 is a schematic illustration of an illumination device adapted inaccordance with the preferred embodiments of the invention;

FIG. 2 is a schematic illustration of a turning film constructed inaccordance with a preferred embodiment of the present invention;

FIG. 3 is a schematic illustration of a turning film constructed inaccordance with an alternate preferred embodiment of the presentinvention;

FIG. 4 is a diagram illustrating light rays propagating in an unfoldedTIR lightguide;

FIG. 4a is a schematic illustration of light rays exiting a lightguideand being turned by a turning lens;

FIG. 5 is a diagram similar to FIG. 4 further illustrating theprinciples of the present invention;

FIG. 6 is a schematic illustration of a turning film constructed inaccordance with still another alternate preferred embodiment of thepresent invention; and

FIG. 7 is a schematic illustration of an illumination device adapted inaccordance with an alternate preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described in terms of several preferredembodiments, and particularly, in terms of a turning film suitable foruse in a backlighting system typically used in flat panel displaydevices, such as a laptop computer display or a desktop flat paneldisplay. The invention, however, is not so limited in application andone of ordinary skill in the art will appreciate that it has applicationto virtually any optical system, for example, to projection screendevices and flat panel televisions. Therefore, the preferred embodimentsdescribed herein should not be taken as limiting of the broad scope ofthe invention.

Referring to FIG. 1, an illumination system 10 includes opticallycoupled a light source 12; a light source reflector 14; a lightguide 16with an output surface 18, a back surface 20, an input surface 21 and anend surface 22; a reflector 24 adjacent the back surface 20; a firstlight redirecting element 26 with an input surface 28 and an outputsurface 30; a second light redirecting element 32; and a reflectivepolarizer 34. The lightguide 16 may be a wedge or a modificationthereof. As is well known, the purpose of the lightguide is to providefor the uniform distribution of light from the light source 12 over anarea much larger than the light source 12, and more particulary,substantially over an entire area formed by output surface 18. Thelightguide 16 further preferably accomplishes these tasks in a compact,thin package.

The light source 12 may be a CCFL that is edge coupled to the inputsurface 21 of the lightguide 16, and the lamp reflector 14 may be areflective film that wraps around the light source 12 forming a lampcavity. The reflector 24 backs the lightguide 16 and may be an efficientback reflector, e.g., a lambertian or a specular film or a combination.

The edge-coupled light propagates from the input surface 21 toward theend surface 22, confined by TIR. The light is extracted from thelightguide 16 by frustration of the TIR. A ray confined within thelightguide 16 increases its angle of incidence relative to the plane ofthe top and bottom walls, due to the wedge angle, with each TIR bounce.Thus, the light eventually refracts out of each of the output surface 18and the back surface 20 because it is no longer contained by TIR. Thelight refracting out of the back surface 20 is either specularly ordiffusely reflected by the reflector 24 back toward and largely throughthe lightguide 16. The first light redirecting element 26 is arranged toredirect the light rays exiting the output surface 18 along a directionsubstantially parallel to a preferred viewing direction. The preferredviewing direction may be normal to the output surface 18, but will moretypically be at some angle to the output surface 18.

As shown in FIG. 2, the first light redirecting element 26 is a lighttransmissive optical film where the output surface 30 is substantiallyplanar and the input surface 28 is formed with an array 36 of prisms 38,40 and 42. The second light redirecting element 32 may also be a lighttransmissive film, for example a brightness enhancing film such as the3M Brightness Enhancement Film product (sold as BEFIII) available fromMinnesota Mining and Manufacturing Company, St. Paul, Minn. Thereflective polarizer 34 may be an inorganic, polymeric, cholestericliquid crystal reflective polarizer or film. A suitable film is the 3MDiffuse Reflective Polarizer film product (sold as DRPF) or the SpecularReflective Polarizer film product (sold as DBEF), both of which areavailable from Minnesota Mining and Manufacturing Company.

With more particular reference to FIG. 2, within array 36, each prism38, 40 and 42 may be formed with differing side angles as compared toits respective neighbor prisms. That is, prism 40 may be formed withdifferent side angles (angles A and B) than prism 38 (angles C and D),and prism 42 (angles E and F). As shown, prisms 38 have a prism angle,i.e., the included angle, equal to the sum of the angles A and B.Similarly, prisms 40 have a prism angle equal to the sum of the angles Cand D, while prisms 42 have a prism angle equal to the sum of the anglesE and F. While array 36 is shown to include three different prismstructures based upon different prism angle, it should be appreciatedthat virtually any number of different prisms may be used.

Prisms 38, 40 and 42 may also be formed with a common prism angle butwith a varied prism orientation. A prism axis “l” is illustrated in FIG.2 for prism 38. The prism axis l may be arranged normal to the outputsurface 30, as shown for prism 38, or at an angle to the output surfaceeither toward or away from the light source as illustrated by phantomaxes “l₊” and “l⁻”, respectively, for prisms 40 and 42.

Prisms 38, 40 and 42 may be arranged within array 36 as shown in FIG. 2in a regular repeating pattern or clusters 43 of prisms, and while thearray 36 is not shown to have like prisms adjacent like prisms, such aconfiguration may also be used. Moreover, within the array 36, theprisms 38, 40 and 42 may change continuously from a first prismconfiguration, such as prism configuration 38, to a second prismconfiguration, such as prism configuration 40, and so on. For example,the prism configuration may change in a gradient manner from the firstprism configuration to the second prism configuration. Alternatively,the prisms may change in a step-wise manner, similar to theconfiguration shown in FIG. 2. Within each cluster 43, the prisms have aprism pitch, which is selected to be smaller than the spatial ripplefrequency. Likewise, the clusters may have a regular cluster pitch.

While the array 36 shown in FIG. 2 has prisms having a symmetricconfiguration, an array of prisms, such as array 36′ shown in FIG. 3formed in light redirecting element 26′, may be used. Referring then toFIG. 3, in the array 36′, prisms 38′, for example, has angle A′ unequalto angle B′. Similarly for prisms 40′ and 42′, angle C′ is unequal toangle A′ and angle D′, and angle E′ is unequal to either of angle A′,angle C′ or angle F′. The array 36′ may be advantageously formed using asingle diamond cutting tool of a predetermined angle, and tilting thetool for each cut producing prisms of differing prism angle andsymmetry. However, it will be appreciated that with the use of a singlecutting tool, the prism angles will be the same, i.e., A+B=C+D=E+F.

It is contemplated that as few as two different prism configurations maybe used and arranged in clusters within the array 36, although as manyprism sizes as necessary to accomplish a modification of the outputprofile from the lightguide 16 may be used. One purpose of the prismside angle variation is to spread and add variable amounts of opticalpower into the first light redirecting element 26. The varyingconfiguration of prisms 38, 40 and 42 serves to provide substantiallyuniform sampling of the input aperture of the lightguide, whichminimizes non-uniformities in the light extracted from the lightguide16. The net result is an effective minimization of the ripple effectparticularly near the entrance end 21 of the lightguide 16.

Referring to FIG. 4 there is shown an unfolded view of a light pathconfined between two non-parallel mirror surfaces, for example, the TIRmirror surfaces in a wedge lightguide. One can represent a rayundergoing multiple TIR reflections as a straight line passing throughmultiple images of the lightguide surfaces. The fan-like structure isthus constructed from a series of wedge cross-sections stacked together,where the left boundary consists of the input surface of the lightguideand its reflected images. As light rays move away from the inputsurface, the confinement angle subtended at each mirror plan increasesby an amount equal to the wedge angle. The top boundary represents alight extraction surface, either the top or bottom of the lightguide,where light emerges from the lightguide into air by the frustration ofTIR. Viewing the lightguide in this manner, the light paths are straightand the number of TIR reflections is easy to count. It also permits oneto trace an extracted ray back to its point of origin on the inputpupil.

Light emits from the wedge over a range of angles starting at about 50°up to about 23°. The lower number is the onset of frustrated TIRtransmission, just after the confinement angle exceeds the TIR limit.The larger angle represents the cutoff of intensity due to depletionfrom multiple bounces at increasing confinement angles. The preciserange of extraction angles depends upon the wedge angle and opticalindex.

In FIG. 4a, light rays 403 and 404 are extracted and then turned towardan observer by a turning lens having identical contiguous prisms 401.Light ray 403 emerges at about 5 relative to the surface (85° fromnormal), approximately the lower limit of emergent light pattern, and isturned by prism 401 into ray path 405, approximately −7.5° to thenormal. Light ray 404 emerges at about 23 (67° from normal), the upperlimit of the emergent light pattern, and is turned by prism 401 into raypath 406, approximately 10 to the normal.

If light rays 403 and 405 emit from points B and A, respectively, on theemission surface, then an observer 407 positioned at the vertex of lightrays 405 and 406 sees the full range of the emitted light patternbetween points A and B. Lightrays 401 and 402, internal to thelightguide, point back to the input surface of the wedge. We see thatthe points of origin for these external rays are separated by 20 inputpupil widths on the left boundary of the fan. Hence, as the observer 407scans from point A to point B, he sees 20 images of the wedge-inputpupil. If the light coupling, from the lamp to the input pupil, isnon-uniform, either spatially or in angle, then the non-uniformity isobserved as a ripple pattern.

The present invention considers multiple prism configurations in acontiguous turning lens and the effect of such structures on opticaloutput uniformity. For example, the turning film may have clusters of 3prism configurations each with a prism angle of 68.88, but havingdifferent prism axis orientations of −1.5°, 0° and +1.5° relative to theturning lens normal. Three rays drawn from an observer, one into eachprism of a cluster, would trace back through the unfolded light diagramto different positions on the input surface. Referring to FIG. 5, threelocations “A”, “B” and “C” are illustrated on the output surface of thewedge. At each location a pencil of 3 rays is drawn from the outputsurface to the fan of the input surface pupils. As observed in FIG. 5,the pencil of rays covers a full pupil width even from position “A”,which would be nearest to the lamp end. Thus, there is providedclose-packed sampling of the pupil that results in a smoothing of thelocal display brightness. The optimal design of the prism configurationwould depend upon details of the wedge geometry and optical index.

So far the clusters 43 of prisms have been described as having asubstantially similar configuration. That is, the same number of prismhaving the same distribution of prism configurations. This is not arequirement, and the clusters 43 may vary in configuration. Thisarrangement may reduce the possibility that at certain locations and forcertain angles aperture sampling may occur at very nearly identicalaperture locations, although at slightly different input angles. Thiscould lead to localized interference type of patterns similar to ripple.

The prism angles can range from 30°-120°, but may be from 40°-75° andmay further be from 60°-75°. The angular difference between prismswithin a cluster may be about 1°-10° and may further be about 1°-5°. Thespacing of the prisms, i.e., the prism pitch, as discussed should beless than the spatial frequency of the ripple and may be anywhere from 1μm-1000 μm, and may still be 20 μm-100 μm and may further be 30 μm-50μm. The array pitch for the array 36, with 50 μm prism pitch, may beapproximately 150 μm. The prism configurations are also influenced bythe refractive index of the optically transmissive film due to theeffects on the refraction face and the TIR at the reflection face.Typical film material may have an index in the range of 1.35-1.75. Forexample, for a polycarbonate material with an index of 1.586, and alightguide with a nominal light exit (glancing) angle of 13°, a prismconfiguration with a nominal included angle of 68.9° may be used.

Referring now to FIG. 6, a turning film 44 formed in accordance with analternate embodiment of the invention includes an array of prisms 45 onan input surface 46. The array of prisms 45 has prisms 47 of at least afirst configuration and a second configuration for providing a ripplereducing effect. Furthermore, the prisms 47 are formed with a curvedfacet reflective side 48. A prism angle is defined as between a tangentto the reflective side 48 and the refractive side 49. As described abovewith respect to the array 36, to reduce the ripple effect the prisms arearranged in clusters within the array 44. Within the clusters, the prismangles may vary and/or the prism orientation, i.e., the angle the prismaxis makes to normal, may vary. It should be appreciated that bothsurfaces may be formed to have a curved facet configuration.

Films including an array of prisms, such as the arrays 36, 36′ or 44,may be formed by extrusion or by a cast and cure process. A master isfirst made by diamond turning the array structure into a surface of acylindrical blank, or roll. One technique for forming the arraystructure in the surface is known as thread cutting. A first thread iscut the length of the roll, and the first thread corresponds to thefirst prism configuration. Starting at a position adjacent the firstthread a second thread is cut. The second thread is cut using adifferent cutting tool or by after adjusting the cutting tool in orderto produce the second prism configuration. This process is repeated,changing cutting tools and/or using multiple tools, until all of thedesired prism configurations are formed.

Alternatively, using an adjustable cutting tool, the tool may beadjusted during the thread cutting process. A single thread is cut, andthe tool is continuously rotated during the cutting process tocontinuously vary the prism configuration such that prisms are formedwithin the array having differing prism configurations.

Still another alternative is to plunge cut the prism configurations.Plunge cutting may be accomplished using multiple cutting tools, oneeach for each prism configuration. An adjustable cutting tool may beused and the tool adjusted for cutting each prism configuration.

Another forming possibility includes the use of a fast tool servo (FTS)actuator. An FTS actuator is shown and described in the commonlyassigned U.S. patent application Ser. No. 09/543,304 now U.S. Pat. No.6,354,709 entitled “Optical Film”, and U.S. patent application Ser. No.09/246,970 now U.S. Pat. No. 6,322,236 entitled “Optical Film WithDefect-Reducing Surface and Method of Making Same” the disclosures ofwhich are hereby expressly incorporated herein by reference. The FTSadvantageously allows for the incorporation of anti-wetout and/or otherproperties into the array 36 as described in the afore-mentioned USpatent applications. Wet-out is an optical defect that appears when twoadjacent optical surfaces within the display device come into contactwith one another. To prevent wet-out, and as discussed in theaforementioned US patent applications, randomly distributed peaks andvalleys, local maxima and minima, may be formed on one or both adjacentsurfaces to minimize the amount and regularity of contact between theadjacent surface. Thus, the input surface 28 of the first lightredirecting element 26 may be formed to include anti wet-out structureas may the output surface 30.

Referring now to the illumination device 50 illustrated in FIG. 7, alightguide 52 forms a portion of the illuminating device 50 and includesa first entrance surface 54, a second entrance surface 56, an outputsurface 58 and a back surface 60. A first light source 62 includes afirst light source reflector 64 and couples to the first entrancesurface 54. A second light source 66 includes a second light sourcereflector 68 and couples to the second entrance surface. Disposedadjacent the output surface 58 is a first light redirecting element 70.The first light redirecting element 70 includes an input surface 72 andan output surface 74. The illumination device 50 further includes areflector 76 adjacent the back surface 60, a second light redirectingelement 78 and a reflective polarizer 80. The lightguide 52 may be awedge or modification thereof. As is well known, the purpose of thelightguide 50 is to efficiently spread light from both light sources 62and 66 over an area much larger than the sources themselves, and moreparticulary, substantially over an entire area formed by output surface58.

The first light redirecting element 70 is preferably a lighttransmissive film the input surface 72 of which is formed with an arrayof prisms in a manner similar to that described above with respect tothe array 36 of the first light redirecting element 26. As theillumination device 50 has a light source disposed on opposite endsthereof, the prisms forming the array may be configured symmetrically orasymmetrically and with differing prism angles. This is to ensure thatlight rays entering the lightguide 52 from either entrance surface 54and 56 are turned along a desired viewing axis. The array of prisms actto redirect the light output from the lightguide 52 along a preferredviewing direction, and further act to reduce the perception of ripple.

As a result of illuminating the lightguide 52 from two sides, an opticaldefect caused by the light rays entering from the opposite entrancesurface 54 and 56 may occur at a center portion 82 of the lightguide 52.The array of prisms formed on the input surface 72 of the lightredirecting structure 70 may be altered at a corresponding centerportion 84 to reduce this perceived optical defect in a manner similarto that by which the array reduces perceived ripple.

Modifications and alternative embodiments of the invention will beapparent to those skilled in the art in view of the foregoingdescription. This description is to be construed as illustrative only,and is for the purpose of teaching those skilled in the art the bestmode of carrying out the invention. The details of the structure andmethod may be varied substantially without departing from the spirit ofthe invention, and the exclusive use of all modifications which comewithin the scope of the appended claims is reserved.

What is claimed is:
 1. An optical turning film comprising: a firstsurface; an array of prisms formed in the first surface, wherein thearray of prisms comprises: a plurality of first prisms, each of thefirst prisms having a first prism configuration, and a plurality ofsecond prisms, each of the second prisms having a second prismconfiguration different from the first prism configuration; a secondsurface opposing the first surface; and wherein light rays incident tosaid first surface are refracted and reflected by the plurality of firstprisms and the plurality of second prisms along a preferred viewingaxis.
 2. The optical turning film as recited in claim 1, wherein theplurality of first prisms and the plurality of second prisms arearranged in clusters within the array.
 3. The optical turning film asrecited in claim 1, wherein the plurality of first prisms comprise atleast one group of first prisms having the first prism configuration andthe plurality of second prisms comprise at least one group of secondprisms having the second prism configuration formed in the first surfaceadjacent the at least one group of first prisms.
 4. The optical turningfilm as recited in claim 3, wherein the plurality of first prismscomprise at least another group of first prisms having the first prismconfiguration formed in the first surface adjacent the at least onegroup of second prisms.
 5. The optical turning film as recited in claim2, wherein the first plurality of prisms are interleaved with the secondplurality of prisms.
 6. The optical turning film as recited in claim 1,wherein the first prisms have a first asymmetric configuration and thesecond prisms have a second asymmetric configuration different from thefirst asymmetric configuration.
 7. The optical turning film as recitedin claim 1, wherein the first prisms have a first prism angle and thesecond prisms have a second prism angle different from the first prismangle.
 8. The optical turning film as recited in claim 7, wherein thefirst prism angle and the second prism angle are in the range from 30°to 120°.
 9. The optical turning film as recited in claim 1, wherein thefirst plurality of prisms and the second plurality of prisms are spacedwith a prism pitch, and wherein the prism pitch is in the range from 1μm to 1000 μm.
 10. The optical turning film as recited in claim 9,wherein the first plurality of prisms and the second plurality of prismsare arranged in clusters and wherein the clusters have a cluster pitch.11. The optical turning film as recited in claim 1, wherein the arraycomprises a transition between one of the plurality of first prisms andone of the plurality of second prisms, the transition comprising asubstantially continuous reconfiguration from the first prismconfiguration to the second prism configuration.
 12. The optical turningfilm as recited in claim 11, wherein the plurality of first prisms andthe plurality of second prisms are arranged in clusters and wherein eachcluster includes at least one said transition.
 13. The optical turningfilm as recited in claim 1, wherein at least one of the first prismconfiguration and the second prims configuration comprises a curvedfacet.
 14. The optical turning film as recited in claim 1, wherein thefirst prism configuration and the second prism configuration eachinclude randomly distributed peaks and valleys.
 15. An optical turningfilm comprising: a sheet of light transmissive optical film having afirst surface and a second surface; a plurality of first lightredirecting prisms formed in the first surface; a plurality of secondlight redirecting prisms formed in the first surface; the plurality offirst light redirecting prisms and the plurality of second lightredirecting prisms arranged in an array of prisms on the first surface;the first light redirecting prisms having a first prism configuration;the second light redirecting prisms having a second prism configuration,different from the first prism configuration; the plurality of firstlight redirecting prisms and the plurality of second light redirectingprisms being organized into a plurality of first prism groups and aplurality of second prism groups; and wherein the first prism groups andthe second prism groups are arranged in the array in a pattern of firstprisms and second prisms.
 16. The optical turning film recited in claim15, wherein the plurality of first prisms and the plurality of secondprisms respectively have prism peaks and wherein the prism peaks arealigned with a longitudinal axis of the sheet.
 17. The optical turningfilm recited in claim 15, wherein the first prism groups and the secondprism groups are interleaved.
 18. The optical turning film recited inclaim 15, wherein the first prisms have a first asymmetric configurationand a second prism configuration different than the first asymmetricconfiguration.
 19. The optical turning film recited in claim 15, whereinthe first prisms have a first prism angle and the second prisms have asecond prism angle different than the first prism angle.
 20. The opticalturning film recited in claim 19, wherein the first prism angle and thesecond prism angel are in a range from 30° to 120°.
 21. The opticalturning film recited in claim 15, wherein the first prisms and thesecond prisms are spaced with a prism pitch, and wherein the prism pitchis in a range from 1 μm to 1000 μm.
 22. An illumination devicecomprising: a lightguide, the lightguide having an input surface and anoutput surface, wherein light rays projected on the input surface areextracted at the output surface by frustrated total internal reflectionat a glancing angle to the output surface; a light source coupled to theinput surface to project the light rays on the input surface; a turningfilm, the turning film comprising a first surface and a second surfaceopposing the first surface, and wherein the first surface is opticallycoupled to the output surface of the lightguide; and an array of prismsformed in the first surface of the turning film, the array of prismscomprising a plurality of first prisms having a first prismconfiguration and a plurality of second prisms having a second prismconfiguration different from the first prism configuration, the array ofprisms arranged to redirect the light rays from the glancing angle to anangle substantially aligned with a viewing axis of the illuminationdevice.
 23. The illumination device as recited in claim 22, wherein thesecond surface is optically coupled to a display device.
 24. Theillumination device as recited in claim 22, further comprising a lightredirecting structure disposed between the turning film and the displaydevice.
 25. The illumination device as recited in claim 22, wherein theturning film comprises a sheet of optically transmissive film.
 26. Theillumination device as recited in claim 22, wherein the first prisms andthe second prisms are arranged in a pattern of prisms within the array.27. The illumination device as recited in claim 22, wherein the firstprisms and the second prisms are substantially aligned with alongitudinal axis of the turning film.